Peel test metrology for solder joint reliability of FCBGA packages

Abstract

A brittle solder joint at the interface between a solder ball and a substrate metal pad could result in an open joint failure after the solder ball is attached to a mothehoard. Esisting metrologies_ such as ball shear. do not always detect the weak solder joint strength. A pecl test for solder joint strength evaluation \vas developed. Several problem were observed during tlic initial stages of the developmnent for this metrology. Tlicsc included solder bridging and inconsistency of the lest results. Soldcr bridging was reduced by changing the stencil design to have stencil openings that match die solder ball hyout and using a jig Io align the coupon and substrate wlule assembling the peel test samples. The paste printing and peeling methods were modified and tlie coupon qualit?. was carefully monitored to inake the sample preparation and testing inore consistent. The impact of the pliysicai aging and reflow on the solder joint strength was invcsligated. It sllowed that tlie peel strength did not decrease with physical aging. but did decrease with an increased number of reflows. The peel test results correlated witti the perfommnce of the substrates. The substrate with low peel strength had weak solder joints and failed under use conditions. The substrates with big11 peel strength dcnionstmted strongcr solder joints and perfomicd well under use conditions. The n~ctrolog! provides a gross check about tlie presence of brittle soldcr joints on FCBGA substrates and is successfully used in monitoring the solder joint strength.